It has recently been proposed, as part of third generation partnership project (3GPP) Release 8 work item applicable to high speed packet access (HSPA) systems, that wireless transmit/receive units (WTRUs) are allowed to transmit on an enhanced dedicated channel (E-DCH) in the CELL_FACH state or idle mode. The WTRUs are allowed to tap E-DCH resources without transitioning to CELL_DCH state, which is referred to as enhanced random access channel (E-RACH) access or E-DCH in CELL_FACH or idle mode.
An E-RACH access is a combination of a random access channel (RACH) preamble transmission phase and an E-DCH transmission phase. FIG. 1 shows an E-RACH access procedure. The RACH preamble transmission phase uses a subset of R99 RACH signatures that a Node B has designated or broadcast for use in E-RACH. The reception of a preamble by the Node B is acknowledged in an acquisition indication channel (AICH), which also assigns a WTRU with an index for a shared E-DCH resource to use. The shared E-DCH resources are pre-designated by the Node B for use in an E-RACH access in CELL_FACH state or idle mode. For all shared E-DCH resources, the parameters are provided to the WTRU during initial setup or broadcast to WTRUs in the cell by the Node B. Each E-DCH resource is associated with an index which is transmitted as part of the acknowledgement for the E-RACH access, or using some other signaling mechanism. Once the WTRU receives the index value, all configuration parameters related to the assigned shared E-DCH resource are known and the WTRU may start transmitting after a possible synchronization period.
In E-DCH (Release 6 and above), the WTRU selects the number of information bits to transmit at every transmission time interval (TTI) based on a set of pre-defined rules. Conceptually, this procedure includes a number of steps. First, the WTRU determines the amount of power it can use for data transmission. To this end, the WTRU measures its power headroom, which is defined as the ratio between the maximum transmission power and the power of the dedicated physical control channel (DPCCH). The maximum transmission power is a known parameter at the WTRU. It is either determined by the WTRU category or signaled by the network. Thus, whenever the WTRU has an estimate of the DPCCH power, the WTRU can calculate the power headroom estimate. In this context power headroom estimation and DPCCH power estimation have a direct relationship. Since the DPCCH power is subjected to variations at every radio slot in response to power control commands from the network, the WTRU filters the DPCCH slot-wise power estimates over a period of one TTI, (i.e., 3 radio slots for 2 ms TTI and 15 radio slots for 10 ms TTI). In a second step, the WTRU uses this power headroom to determine the set of transport format that can be used for transmission, also referred to as the set of supported E-DCH transport format combinations (E-TFCs). An E-TFC that is in the set of supported E-TFCs is said to be in supported state. This step may be referred to as E-TFC restriction. Finally, the WTRU determines how many bits will be transmitted in the coming TTI from each MAC-d flow (up to the maximum supported E-TFC) based on the serving grant, non-scheduled grant, reference E-TFCs, hybrid automatic repeat request (HARQ) profiles, multiplexing list, etc. This step is referred to as E-TFC selection in the 3GPP specifications.
When the WTRU initiates E-DCH transmission in CELL_FACH state or idle mode, the WTRU may not know the power headroom or may not have a sufficiently accurate power headroom estimate to carry out the E-TFC selection functions and protocol data unit (PDU) creation procedures in time for the start of the E-DCH dedicated physical data channel (E-DPDCH) transmission. Therefore, it would be desirable to provide a method for power headroom estimation that would allow the WTRU to perform E-TFC selection functions and PDU creation procedures before that point. It is further desirable to provide such an estimation method for reporting UE transmission power headroom (UPH) measurements when the scheduling information (SI) containing the UPH is transmitted before the measurement is considered sufficiently reliable.